A photocopy machine typically employs two different types of lamps, one being referred to as an exposure lamp and the other as a fusing lamp. The exposure lamp is purely for light emitting purposes during the exposure phase of operation. The fusing lamp on the other hand is primarily for heating purposes to "set" the toner employed in the photocopy machine. In accordance with the present invention, the principles thereof are applied primarily in connection with a fusing lamp, but may also be applied to other general heating purposes.
Fusing lamps are typically of single filament construction and have a length corresponding to the maximum size (length) of paper that is to be reproduced. One infrared lamp, in particular, has utilized two filaments disposed and electrically connected in parallel within a quartz envelope in order to allow substantially higher operating wattages to be achieved by simultaneously energizing both filaments (see U.S. Pat. No. 3,443,144 to Robert Freeze). According to Freeze, the use of parallel filaments provides higher heat density per unit area of envelope wall. The lamp in Freeze, as described, does not provide for selective activation of the filaments (i.e. dual level fusing), to adjust for different incremental wattage output requirements. Finally, the Freeze lamp provides a series of spacers disposed along the length of the filaments to prevent them from sagging against the quartz envelope, but does not isolate the filaments from each other to prevent shorting.
In some photocopier applications it would be desirable to have different levels of energy available for fixing the toner (i.e. fusing) onto the copy of the original document. A high level of energy is desirable at the time of starting to make the first copy, as at that point the fusing system is cold. Typically, some time is required to bring the fusing system up to operating temperature and may limit the speed in which the first copy is supplied, which is one measurement criteria used in photocopying machine evaluation. In U.S. Pat. No. 4,442,374 to Morris et al, a dual length filament incandescent lamp is provided that allows for two different lengths of fusing usually at a prescribed amount of energy per length in order to set toner on short paper and long paper equally, i.e., energy per square centimeter of paper would be the same for both short and long paper. This then will result in two levels of total lamp energy, but one level of fusing energy per length of paper. The specification of Morris et al (U.S. Pat. No. 4,442,374) is hereby incorporated by reference.
It is believed, therefore, that a tubular incandescent lamp providing multiple levels of energy for heating applications while promoting compactness and manufacturing ease would constitute a significant advancement in the art.